Signal Watermarking in the Presence of Encryption

ABSTRACT

A method is disclosed that enables the transmission of a digital message along with a corresponding information signal, such as audio or video. The supplemental information contained in digital messages can be used for a variety of purposes, such as enabling or enhancing packet authentication. In particular, a telecommunications device that is processing an information signal from its user, such as a speech signal, encrypts the information signal by performing a bitwise exclusive-or of an encryption key stream with the information signal stream. The device, such as a telecommunications endpoint, then intersperses the bits of the digital message throughout the encrypted signal in place of those bits overwritten, in a process referred to as “watermarking.” The endpoint then transmits the interspersed digital message bits as part of a composite signal that also comprises the encrypted information bits. No additional bits are appended to the packet to be transmitted, thereby addressing compatibility issues.

FIELD OF THE INVENTION

The present invention relates to telecommunications in general, and,more particularly, to transmitting a digital message along with aninformation signal.

BACKGROUND OF THE INVENTION

Modern telecommunications systems feature the routing of mediainformation signals, such as audio or video, over one or morepacket-based networks, such as the Internet. In Voice over InternetProtocol (or “VoIP”), for example, voice signals from the voiceconversations to be routed are digitized and formatted into datapackets, which are then transmitted through the network. Atelecommunications network that is based on VoIP is able to transmitvoice conversations between telecommunications endpoints that are ableto access the network.

Each telecommunications endpoint, whether voice-capable or not, is apacket-based device that is capable of exchanging information with otherdevices; the endpoint exchanges information in a manner similar to how apersonal computer is able to exchange information with other computersthroughout the Internet. Consequently, the endpoint is vulnerable tomany of the same or similar packet attacks as is a personal computer,such as “Denial-of-Service” (DoS) attacks. In fact, there are manysources of potential packet attacks that can be directed at an endpointfrom within any of a variety of networks that are interconnected to thenetwork used by the endpoint.

To improve the ability of the endpoint to withstand packet attacks, sometype of authentication is necessary. Authentication enables the endpointto decide which of the arriving packets are legitimate and which shouldbe discarded. A standard protocol known as Secure Real-time TransportProtocol (SRTP) describes the procedures for performing one method ofauthentication. However, there is a drawback to this protocol. In orderto authenticate a packet, it is necessary to compute a message digestover the header and the payload of the packet. This computation requiresa significant amount of processing at the endpoint and can possiblyoverload the endpoint's processor.

Simpler schemes for authenticating each packet are available thatrequire fewer processing resources. However, because of restrictionsspecified by SRTP and firewall behavior in the networks, it is typicallynot possible to append the additional information needed by the simplerschemes. Additionally, other applications unrelated to authenticationcan require the transmission of supplemental information, such as bitsto convey additional control information for a particular feature. Theproblem is that unused bit positions in existing messages often do notexist and appended bits often cannot be transmitted, in order to conveythe supplemental information.

Furthermore, due to processing path complexity, knowing where in theprocessing path to consider introducing the supplemental information tobe sent can be challenging. As depicted in FIG. 1, transmit processingpath 100 comprises information compression, as performed by compressor111; encryption of the compressed signal, as performed by encryptor 112;and channel coding of the encrypted signal, as performed by channelcoder 113. With respect to the encryption processing, a block of data tobe encrypted is typically sent through many stages of encryptionoperations that involve secret keys. In this case, every bit of theoutput data is affected by every bit of the input data. Care must betaken as to where supplemental information is added to the processedsignal, as tampering with encrypted data can lead to disastrous resultsduring the decryption of the processed signal at the receive node.

What is needed is a technique to free up additional bit positions ineach packet in a packet stream, for sending digital messages thatcontain supplemental information related to authentication or otherpurposes, while maintaining the integrity of the processed signal andwithout some of the disadvantages in the prior art.

SUMMARY OF THE INVENTION

The present invention enables the transmission of a digital messagealong with a corresponding information signal, such as audio or video.The supplemental information contained in digital messages can be usedfor a variety of purposes, such as enabling or enhancing packetauthentication. In particular, a telecommunications device that isprocessing an information signal from its user, such as a speech signal,encrypts the information signal by performing a bitwise exclusive-or ofan encryption key stream with the information signal stream. The device,such as a telecommunications endpoint, then intersperses the bits of thedigital message throughout the encrypted signal in place of those bitsoverwritten, in a process referred to as “watermarking.” The endpointthen transmits the interspersed digital message bits as part of acomposite signal that also comprises the encrypted information bits. Inthis way, no additional bits are appended to the packet to betransmitted, thereby addressing the issue of compatibility with existingprotocols and firewalls.

What is different about the technique of the illustrative embodiment, ascompared to some techniques in the prior art, is that the watermarkingoccurs after the encryption process. The key to the successful combiningof the supplemental bits in the digital message with the encrypted mediainformation bits is in the bitwise exclusive-or operation that isperformed. This operation guarantees that only one bit in the encryptedstream affects only the corresponding bit in the decrypted informationstream. This is because the decryption process in the receiving node issimply the bitwise exclusive-or with the same key stream used by thetransmitting node. Watermarking after encryption is advantageous in thatfor some digital processing paths, it is easier to append thewatermarking stage near the end of the path, instead of having to embedthe watermarking between two existing stages (i.e., information codingand encryption) of the digital signal processing.

In some embodiments of the present invention, a message digest that canbe used for authenticating the transmitted packet at the receiving nodeis also computed and transmitted. The digest is computed after thewatermarking has taken place, which ensures that the modified messagewill be considered authentic by the receiving node when it performsauthentication processing. Advantageously, the use of the messagedigest, in combination with the watermarking of the media informationstream with authentication-related digital messages, can enhance theresiliency of a receiving endpoint to packet attacks.

The illustrative embodiment of the present invention comprises:encrypting an information signal, resulting in an encrypted signal thatis M bits in length; substituting N bits of the encrypted signal with atleast a portion of a digital message, the substitution of the N bits ofthe encrypted signal resulting in a composite signal; and transmittingthe composite signal to a receiving node; wherein M and N are positiveintegers and N is less than M

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts transmit processing path 100 in the prior art.

FIG. 2 depicts a schematic diagram of telecommunications system 200, inaccordance with the illustrative embodiment of the present invention.

FIG. 3 depicts a block diagram of transmit processing path 300 oftelecommunications endpoint 202-m, in accordance with the illustrativeembodiment of the present invention.

FIG. 4 depicts a block diagram of receive processing path 400 oftelecommunications endpoint 202-m, in accordance with the illustrativeembodiment of the present invention.

FIG. 5 depicts a flowchart of the salient tasks that pertain to theprocessing of an information signal along transmit processing path 300.

FIG. 6 depicts a flowchart of the salient tasks that pertain to theprocessing of an information signal along receive processing path 400.

DETAILED DESCRIPTION

FIG. 2 depicts a schematic diagram of telecommunications system 200, inaccordance with the illustrative embodiment of the present invention.System 200 routes voice conversations, or other types of mediainformation signals such as video and other types of audio (e.g., music,etc.), between network elements such as telecommunications endpoints.System 200 comprises: packet transmission network 201;telecommunications endpoints 202-1 through 202-M; and gateways 203-1through 203-N, wherein M and N are positive integers. All of theelements depicted in FIG. 2 are interconnected as shown.

Packet transmission network 201 is used to transport one or more typesof media, such as Voice over Internet Protocol (or “VoIP”), for thesubscribers of a service provider. Network 201 comprises one or moretransmission-related nodes such as routers that are used to direct datapackets that carry processed information signals (e.g., voice packets,etc.) from one or more sources to the correct destinations of thosepackets. Network 201 is capable of handling Internet Protocol-basedmessages that are transmitted among the network elements that haveaccess to network 201, such as the various telecommunications endpointsand gateways throughout system 200. Although network 201 in theillustrative embodiment is a Voice-over-IP service provider's network,network 201 could alternatively be the Internet, some other type ofInternet Protocol-based network, or some other type of packet-basednetwork.

In some embodiments, network 201 comprises one or more local areanetworks (or “LAN”), which provide for the local distribution ofsignals, such as in an enterprise system. For example, each local areanetwork can enable one or more telecommunications endpoints to access awider network. Each local area network comprises networking equipmentsuch as hubs, bridges, and switches, and operates in accordance with anetworking protocol such as Ethernet, IEEE 802.3, IEEE 802.11, and soforth.

Telecommunications endpoint 202-m, for m=1 through M, is a communicationappliance such as a deskset, a conferencing unit, a wireless terminal, adesktop or portable computer (i.e., “softphone”), an Internet phone, andso forth. As a packet-based device, telecommunications endpoint 202-m iscapable of exchanging information with other devices intelecommunications system 200, in a manner that is similar to how apersonal computer is able to exchange information with other computersthroughout the Internet.

Endpoint 202-m is capable of digitizing voice signals from its user andformatting the digitized signals into transmittable data packets throughan audio compressor/decompressor (or “CODEC”) circuit and through anencryptor, as described below and with respect to FIG. 3. Similarly, theCODEC circuit of endpoint 202-m is also capable of receiving datapackets and converting the information contained within those packetsinto voice signals that are understandable by the endpoint's user, asdescribed below and respect to FIG. 4. Furthermore, endpoint 202-m iscapable of performing the tasks described below and with respect toFIGS. 5 and 6, in accordance with the illustrative embodiment of thepresent invention. It will be clear to those skilled in the art, afterreading this specification, how to make and use endpoint 202-m.

Gateway 203-n, for n=1 through N, is a networking device that connectspacket transmission network 201 with the network that is associated witha particular gateway (e.g., the Public Switched Telephone Network, etc.)by forwarding data packets between the two networks. Each gateway 203-nacts as a translator between the two different types of networks towhich it is connected (i.e., packet network 201 and another network).Because gateway 203-n connects two different types of networks together,one of its main functions is to convert between the differenttransmission and coding techniques used across the two networks.Therefore, gateway 203-n is also capable of converting betweencompressed and decompressed signals (e.g., via a “CODEC” circuit, etc.).In some embodiments, gateway 203-n is capable of executing at least someof the tasks described below and with respect to FIGS. 4 and 5. It willbe clear to those skilled in the art, after reading this specification,how to make and use gateway 203-n.

In accordance with the illustrative embodiment, the devices of system200 are capable of wired communications and of operating in a serviceprovider environment. As those who are skilled in the art willappreciate, in some alternative embodiments some or all of the devicesof system 200 are capable of wireless communications, and of operatingin various types of networks (e.g., public, private, etc.). Furthermore,in some alternative embodiments devices other than endpoints or gatewaysare capable of performing the tasks described below and with respect toFIGS. 4 and 5. It will be clear to those skilled in the art, afterreading this specification, how to apply the techniques of theillustrative embodiment to other types of devices and in other operatingenvironments.

FIG. 3 depicts a block diagram of transmit processing path 300 oftelecommunications endpoint 202-m, in accordance with the illustrativeembodiment of the present invention. Transmit path 300 processes aninformation signal, such as a voice signal from the endpoint's user,which can be acquired by a transducer such a microphone. If not alreadyin digital form, analog-to-digital converter 310 converts theinformation signal from analog form to digital form.

After being digitized, information encoder 311 encodes the informationsignal to achieve data compression. In the illustrative embodiment, inwhich the information signal is a voice signal, encoder 311 comprises avocoder, a type of CODEC known in the art, which performs theinformation compression. The vocoder takes the time-series waveform dataand converts the data to digital symbols corresponding to speech patterncharacteristics.

In accordance with the illustrative embodiment of the present invention,encoder 311 operates in accordance with the ITU G.729 protocol standard,as is known in the art. The ITU G.729 protocol standard is described inITU-T Recommendation G.729, “Coding of Speech at 8 Kbit/s usingConjugate-Structure Algebraic-Code-Excited Linear-Predication(CS-ACELP),” March 1996, as well as the corresponding Annexes (i.e.,Annex A, Annex B, and so on), all of which are incorporated herein byreference. In some alternative embodiments, encoder 311 can amodel-based codec other than one that is based on ITU G.729 or awaveform-based codec such as one that is based on ITU G.711.

Encryptor 312 encrypts the compressed signal in well-known fashion,resulting in an encrypted information signal frame. The encryption taskconsists of performing a bitwise exclusive-or of a key stream and thebit stream of the compressed audio signal, frame-after-frame.

Concurrently, transmit controller 313 determines if a supplementalsignal (e.g., for control purposes, etc.) needs to be transmitted alongwith the encoded information signal to the receiving node. For example,a supplemental signal might be an authentication code, which can be usedby the receiving node to authenticate the packets that it receives.Controller 313 either acquires the supplemental signal from an outsidesource or generates the signal itself. When a supplemental signal needsto be transmitted, controller 313 writes to memory device 314 a digitalmessage that represents the supplemental signal.

Digital signal processor 315 receives the compressed and encryptedinformation signal frames from encryptor 312. Processor 315 also readsthe digital message from memory 314 and substitutes a selectedcombination of bits in the encoded information signal with the bits fromthe digital message. In accordance with the illustrative embodiment, thecombination of bits selected is based on prior analysis. In somealternative embodiments, processor 315 determines the combination ofbits by evaluating data from the encoded information signal. In doingso, processor 315 determines which of the information signal bits can besubstituted (i.e., overwritten) with bits received from controller 314as described below, based on one or more characteristics of the encoder,such as the perceptual significance of each bit in the encodedinformation signal. The signal that results from the substitution is acomposite signal that comprises the information signal and supplementalsignal.

Processor 315 also computes a message digest, as is known in the art.The message digest is based on at least a portion of the compositesignal. Processor 315 then includes the message digest as part of thecomposite signal to be transmitted to an endpoint, which can use themessage digest for authentication purposes.

Channel coder 316 prepares the composite signal for transmission bycoding the frame for forward error correction and formatting the framefor transmission. The channel-coded, composite signal is sent totransmitter 317, which then transmits the signal in well-known fashionto network 201.

FIG. 4 depicts a block diagram of receive processing path 400 oftelecommunications endpoint 202-m, in accordance with the illustrativeembodiment of the present invention. Receive path 400 receives packets,each of which comprises one or more composite signal frames, from atransmitting endpoint or from another packet-capable device (e.g.,gateway 203-n, etc.); path 400 then processes the received compositesignal frames. In particular, receiver 409 receives the packet signalsfrom network 201, in well-known fashion. For each received frame,digital signal processor 410 detects and corrects errors, and decryptsthe encrypted bits. Processor 410 also processes the message digest,separates the supplemental bits from the encoded information bits, andstores those bits into memory 411.

Receive controller 412 accesses the supplemental bits as needed. Forexample, if the supplemental bits represent an authentication code,controller 412 uses the authentication code to determine theauthenticity of the received encoded information signal.

Information decoder 413 decodes (decompresses) the encoded informationsignal to achieve a reconstructed version of the original informationsignal. In the illustrative embodiment, in which the information signalis a voice signal, decoder 413 comprises a vocoder, which is a type ofCODEC known in the art and which performs the information decompression.The vocoder takes the digital data present in the received encodedinformation signal, which data correspond to speech patterncharacteristics, and converts the data to time-series waveform data.

In accordance with the illustrative embodiment of the present invention,decoder 413 operates in accordance with the ITU G.729 protocol standard,as is known in the art.

Continuing along receive path 400, digital-to-analog converter 414converts the decoded information signal from digital form to analogform. Afterwards, the analog information signal can be additionallyprocessed for eventual presentation to the receiving endpoint's user,such as by an acoustic speaker.

FIGS. 5 and 6 depict flowcharts of the salient tasks that are executedby telecommunications endpoint 202-m, in accordance with theillustrative embodiment of the present invention. The salient tasks inFIG. 5 pertain to the processing of the information signal alongtransmit processing path 300 depicted in FIG. 3. The salient tasks inFIG. 6 pertain to the processing of the information signal along receiveprocessing path 400 depicted in FIG. 4. For pedagogical purposes, theexample that follows illustrates a call session that is in progress, inwhich endpoint 202-1 is sending a stream of audio packets to endpoint202-2. In the example, transmitting endpoint 202-1 is performing thetasks with respect to FIG. 5; and receiving endpoint 202-2 is performingthe tasks with respect to FIG. 6. Some of the tasks that appear in FIGS.5 and 6 can be performed in parallel or in a different order than thatdepicted, as those who are skilled in the art will appreciate.

In some embodiments, as those who are skilled in the art willappreciate, endpoint 202-2 might be concurrently sending a stream ofaudio packets back to endpoint 202-1—in which case, endpoint 202-2 alsoperforms the tasks with respect to FIG. 5 and endpoint 202-1 alsoperforms the tasks with respect to FIG. 6. Alternatively, as those whoare skilled in the art will also appreciate, other nodes in system 200can perform the tasks depicted in FIGS. 5 and 6. Instead of audiopackets, in some alternative embodiments, the endpoints exchangeinformation signals that convey other than audio information, such asvideo information signals.

Referring to FIG. 5, transmit processing path 300 of endpoint 202-1receives a segment of an audio signal in well-known fashion at task 501.

At task 502, transmit path 300 compresses the audio signal in accordancewith the ITU G.729 protocol standard, providing a compressed audiosignal frame that is M bits in length, wherein M is equal to 80 in thiscase.

At task 503, transmit path 300 encrypts the compressed audio signalframe in well-known fashion, resulting in an encrypted signal frame. Theencryption task consists of performing a bitwise exclusive-or of a keystream and the bit stream of the compressed audio signal,frame-after-frame. Various related techniques for encryption arewell-known in the art and can be applied here. In some alternativeembodiments, transmit path 300 encrypts the signal before compressingthe signal.

At task 504, transmit path 300 obtains a digital message to betransmitted along with the audio signal. For example, controller 313computes an authentication code based on a portion of the audio signalframe, a shared key, and a hashing algorithm. Various other techniquesfor computing an authentication code are well-known in the art and canbe applied here. As those who are skilled in the art will appreciate,the digital message can contain other data to be transmitted, such asclosed-captioning information that is to coincide with the audioinformation signal also being sent.

At task 505, in accordance with the illustrative embodiment, transmitpath 300 substitutes N bits of the M-bit encrypted audio signal with atleast a portion of the digital message, resulting in a composite signalframe. An empirical study made in conjunction with the present inventionshows that in each 80-bit frame of the encoded audio information signalreceived from encoder 211, processor 215 can substitute the bits at oneor more of bit positions 39, 40, 68, 69, 30, and 41 with relatively lowimpact on the perceived audio quality; in the frame, the bits at bitpositions 1 and 80 are the first and last bits, respectively, to betransmitted. As an example, processor 215 might substitute bit numbers39, 40, 68, and 69 for a total of four bits in the frame (i.e., N isequal to four). As a second example, processor 215 might substitute bitnumbers 39, 40, 68, 69, 30, and 41 for a total of six bits in the frame(i.e., N is equal to six).

In the bit stream ordering in the illustrative embodiment, bits 40 and69 correspond to the least significant bits of the fixed-codebook indexbits of the first and second subframes, respectively, within each 80-bitframe, in accordance with the ITU G.729 protocol standard. Furthermore,bits 39 and 68 correspond to the next least significant bits (i.e., areone bit position more significant than bits 40 and 69) of thefixed-codebook index bits of the first and second subframes,respectively. The concepts of the fixed-codebook index bits and thesubframes that are generated are well-known in the art. As those who areskilled in the art will appreciate, the bits that are substituted can beone or more of the least significant, fixed-codebook index bits of oneor more subframes in each generated frame; this applies even if themodel-based audio coder being used operates in accordance with aprotocol standard other than ITU G.729.

At task 506, transmit path 300 computes a message digest based on atleast a portion of the composite signal frame, in well-known fashion. Insome embodiments, the computing of the message digest is performed inaccordance with the Secure Real-time Transport Protocol (SRTP). Transmitpath 300 includes the message digest as part of the composite signalframe.

At task 507, transmit path 300 channel codes the composite signal framein well-known fashion. The channel coding is performed to enable errordetection and correction on the part of receiving endpoint 202-2. Insome alternative embodiments, transmit path 300 performs channel codingbefore the bit substitution described above and with respect to task505.

At task 508, transmit path 300 transmits the composite signal frame toendpoint 202-2. After task 508, task execution proceeds back to task 501to process the next frame's worth of audio information signal.

Referring to FIG. 6, at task 601 receive processing path 400 of endpoint202-2 receives the composite signal frame transmitted by endpoint 202-1.

At task 602, receive path 400 detects and corrects errors in thereceived composite signal frame.

At task 603, receive path 400 authenticates the composite signal frameusing the received message digest or any authenticated-relatedinformation in the received digital message, or both. Receive path 300can compare the received message digest with a computed message digest,where the computed message digest is based on the received informationbits, a shared key, and a hashing algorithm. Similarly, receive path 300can compare the information in the received digital message (e.g., anauthentication code, etc.) with computed data.

At task 604, if the composite signal frame has been authenticated—forexample, the received message digest matches the computed messagedigest—task execution proceeds to task 605. Otherwise, task executionproceeds to task 608.

At task 605, receive path 400 decrypts the composite signal, whichresults in a decrypted signal frame. In accordance with the illustrativeembodiment, the decryption process is a bitwise exclusive-or with thesame key stream that was used by transmitting endpoint 202-1 to encryptthe signal. As a result, except for the bits that were overwritten withthe digital message bits, the bits of the compressed audio signal arerecovered intact.

At task 606, receive path 400 decompresses (i.e., reconstructs) an audiosignal from the decrypted signal frame. In some embodiments, if bitscorresponding to a digital message are present in the decrypted signalframe, the values of some or all of those bits are modified to improvethe results of the information reconstruction.

At task 607, receive path 400 sends the reconstructed audio signal to anaudio circuit for additional processing, in well-known fashion. Taskexecution then proceeds back to task 601 to process the next framereceived from endpoint 202-1.

At task 608, in the case of the composite signal frame not beingauthentic, receive path 400 ignores the received composite signal frame.Task execution then proceeds back to task 601 to process the next framereceived from endpoint 202-1.

It is to be understood that the disclosure teaches just one example ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

1. A method of communicating a digital message with an informationsignal, the method comprising: encrypting said information signal,resulting in an encrypted signal that is M bits in length; substitutingN bits of said encrypted signal with at least a portion of said digitalmessage, the substitution of said N bits of said encrypted signalresulting in a composite signal; and transmitting said composite signalto a receiving node; wherein M and N are positive integers and N is lessthan M.
 2. The method of claim 1 wherein the encryption of saidinformation signal is based on a bitwise exclusive-or operation of saidinformation signal with a key stream.
 3. The method of claim 1 furthercomprising compressing said information signal prior to encryption. 4.The method of claim 3 wherein said information signal comprises an audiosignal, and wherein the compression of said information signal isperformed in accordance with the ITU G.729 protocol standard.
 5. Themethod of claim 1 further comprising channel coding said compositesignal.
 6. The method of claim 1 further comprising channel coding saidencrypted signal prior to the substitution of said N bits.
 7. The methodof claim 1 further comprising computing a message digest that is basedon at least a portion of said composite signal.
 8. The method of claim 7wherein the computing of said message digest is performed in accordancewith the Secure Real-time Transport Protocol.
 9. A method ofcommunicating a digital message with an information signal, the methodcomprising: encrypting said information signal, based on a bitwiseexclusive-or operation of said information signal with a key stream,resulting in an encrypted signal that is M bits in length; substitutingN bits of said encrypted signal with at least a portion of said digitalmessage, the substitution of said N bits of said encrypted signalresulting in a composite signal; and transmitting said composite signalto a receiving node; wherein M and N are positive integers and N is lessthan M.
 10. The method of claim 9 further comprising compressing saidinformation signal prior to encryption.
 11. The method of claim 10wherein said information signal comprises an audio signal, and whereinthe compression of said information signal is performed in accordancewith the ITU G.729 protocol standard.
 12. The method of claim 9 furthercomprising channel coding said composite signal.
 13. The method of claim9 further comprising channel coding said encrypted signal prior to thesubstitution of said N bits.
 14. The method of claim 9 furthercomprising computing a message digest that is based on at least aportion of said composite signal.
 15. The method of claim 14 wherein thecomputing of said message digest is performed in accordance with theSecure Real-time Transport Protocol.
 16. A method of communicating adigital message with an information signal, the method comprising:encrypting said information signal, based on a bitwise exclusive-oroperation of said information signal with a key stream, resulting in anencrypted signal that is M bits in length; substituting N bits of saidencrypted signal with at least a portion of said digital message, thesubstitution of said N bits of said encrypted signal resulting in acomposite signal; computing a message digest that is based on at least aportion of said composite signal; and transmitting said composite signaland said message digest to a receiving node; wherein M and N arepositive integers and N is less than M.
 17. The method of claim 16further comprising compressing said information signal prior toencryption.
 18. The method of claim 17 wherein said information signalcomprises an audio signal, and wherein the compression of saidinformation signal is performed in accordance with the ITU G.729protocol standard.
 19. The method of claim 16 further comprising channelcoding said composite signal.
 20. The method of claim 16 furthercomprising channel coding said encrypted signal prior to thesubstitution of said N bits.
 21. The method of claim 16 wherein thecomputing of said message digest is performed in accordance with theSecure Real-time Transport Protocol.